Conventionally, a waveguide/planar line transducer is used as an interface portion that electromagnetically couples a waveguide and a planar transducer and that changes lines. The waveguide/planar line transducer is for example used by being attached to a circuit board. The circuit board has a planar line that amplifies and frequency converts microwaves or millimeter waves that are transmitted through a waveguide.
As an example of a circuit board that includes a waveguide/planar line transducer, Patent Document 1 (See below) discloses a circuit board that transmits high-frequency signals from a planar line (strip line) to a waveguide. The circuit board that is disclosed in Patent Document 1 has two coupling holes that are oppositely arranged between the waveguide and the planar line in a manner sandwiching a cavity and also mutually electromagnetically coupled.
According to the circuit board disclosed in Patent Document 1, since it is possible to reduce the impedance mismatch of the line that transmits the high-frequency signal by the two coupling holes, it is possible to reduce the passage loss of the high-frequency signal.